In general, I tend not to enjoy any forum that is just busy all the time. That's not really the fault of the users, it's just hard to sustain any sort of discussion, though you could argue that the busiest communities on the web aren't usually for general discussion and tend to focus on something in particular or just random jokes.
I'm going to have to go with YouTube too, mostly going off video comments. The various "debates" I've seen in the past are always so clumsy on either side, with everyone (if not everyone, most people) coming off as uninformed, generalising idiots. It doesn't help that the most linked/watched videos tend to be some sort of extreme, sensationalist thing... but that's the point, right? Yarp yarp. Otherwise it just seems to consist of quotes if the video is a clip from a comedy show or something. To be fair, I've done this before quite a few times, but I don't need to read the same line again and again. Yes, I know it was funny but gewhref!
I used MySpace a few times some years back, and the forums and profiles on there were terrible too. The whole scene just isn't my cup of tea to be honest. Facebook, from what I've seen, doesn't fare much better in some departments, but I'm not being nice to the actual decent people all over the web here. I'm generalising myself.
Yahoo Answers, in my opinion, is quite a useful tool at times if you just have a basic question. It's helped me when it's topped a Google search when I'm troubleshooting or whatever. Yeah, some questions and some answers are mindnumbingly stupid, but you get a bit of humour here and there that makes it quite enjoyable at times. I don't use it that often though.
I should check out some of these suggestions... ;)
